1. Field of the Invention
The present invention relates to a low pressure mercury vapor electric discharge lamp and a method of manufacturing the lamp, and more specifically, to a low pressure mercury vapor electric discharge lamp by which the adhering strength of a fluorescent film and a total beam of light are improved as well as a beam of light maintaining ratio is increased and a blacking is reduced.
2. Description of the Related Art
There are widely used low pressure mercury vapor electric discharge lamps such as fluorescent lamps and the like having a fluorescent material film formed on the inner surface of a glass tube. FIG. 2 shows the general structure of a mercury vapor electric discharge lamp 1. In FIG. 2, the reference numeral 2 denotes a glass tube and numeral 3 denotes a fluorescent material film (phosphor layer) coated on the inner wall of the glass tube 2. In general, the low pressure mercury vapor electric discharge lamp is made by the following method. That is, after a slurry of fluorescent coating composition prepared by mixing a fluorescent material with a predetermined amount of a binder and a coupling agent is coated onto the inner surface of the glass tube, a fluorescent material film is formed on the inner surface of the glass tube integrally therewith through a drying/baking process.
Conventionally, a solution prepared by dissolving nitrocellulose or ethyl cellulose in organic solvent such as butyl acetate, xylene or the like is ordinarily used as the binder and a fluorescent material coating composition made by suspending the fluorescent material and the coupling agent in the organic solvent is coated onto the inner surface of a glass tube (bulb) and dried and baked to form a fluorescent material film. As the coupling agent used to the organic solvent composition, both Ba--Ca boric acid compound is ordinarily used together with calcium pyrophosphate.
When the above organic solvent composition is used, however, a problem arises in that the organic solvent composition is vaporized when it is handled and adversely affects a human body and the safety in job and the quality of products are lowered by it. In particular, when the glass tube (bulb) to which the above composition is coated is baked, harmful gases such as CO, CO.sub.2, No.sub.x, etc. are liable to be generated and may cause environmental pollution such as air pollution and the like.
Further, an electric discharge lamp having a filament in a vacuum glass tube has a problem that a gas is liable to be discharged in the tube by an increase of a temperature in the tube and thus there is a tendency that lamp characteristics are lowered. In particular, there is a problem that a total beam of light is remarkably lowered when the lamp is lit.
Taking the above problems into consideration, the organic solvent composition tends to be gradually not used, whereas a method of using a water soluble fluorescent material coating composition prepared by dissolving a water soluble binder to water is employed. That is, this is a method of preparing a water soluble composition by suspending fluorescent material and a coupling agent to a solution made by dissolving the water soluble binder to water and coating the composition to the inner surface of a glass tube and drying and baking it to thereby form a fluorescent material film.
It is proposed to use at least one of Sr(NO.sub.3).sub.2 and Ba(NO.sub.3).sub.2 and at least one of boric acid, boric acid anhydride and calcium pyrophosphate as the coupling agent to be used when the water soluble composition is formed as disclosed in, for example, Japanese Patent Application Laid-Open No. 58-126658.
However, the coupling agent has a problem that lamp characteristics are liable to be lowered because the coupling agent has not sufficient chemical stability with respect to a fluorescent material. Further, since the coupling agent is composed of a non-light-emitting material of fine particles, if it is added in a large amount, the light emitting characteristics of a fluorescent material film is lowered, thus the light emitting output of a fluorescent lamp and the like and the beam of light maintaining ratio thereof when the lamp is lit are lowered as well as blacking is caused at an earlier time.
On the other hand, when an additive amount of the coupling agent is too small, there is a drawback that the fluorescent material film does not strongly adhere to a glass tube and what is obtained is only a fluorescent material film which is exfoliated or peeled-off very easily. In particular, the fluorescent material film is liable to be exfoliated or fallen away in a process for bending a glass tube to manufacture a circular electric discharge lamp or a high load type electric discharge lamp using the coupling agent or in a process for sealing a nitrogen gas or a rare gas into the glass tube, thus the value of the thus made electric discharge lamp as a commodity is often lost.
In general, although the fluorescent material coating composition using the organic solvent can form a fluorescent material film on the inner surface of a glass tube at a coupling strength higher than that of the composition using a water soluble binder, it is confirmed that the fluorescent material film is also liable to be exfoliated or fallen away in the process for bending the glass tube likewise. Further, when the water soluble coating composition is used, there is a tendency that exfoliation is caused more often in manufacturing processes as compared with the organic coating composition.
As disclosed in the above published application, it is contemplated to increase the concentration of the coupling agent such as Sr(NO.sub.3).sub.2 and Ba(NO.sub.3).sub.2 or boric acid, boric acid anhydride, calcium pyrophosphate and the like as a countermeasure for preventing the exfoliation and falling away of the fluorescent material film. However, an increase of the concentration of the coupling agent causes such adverse effects as a decrease of the beam of light of a lamp and a decrease of the beam of light maintaining ratio thereof, thus it is difficult to obtain an electric discharge lamp excellent in both the adhering strength (coupling force) of the fluorescent material film and lamp characteristics in any case.
An object of the present invention made to solve the above problems is to provide a low pressure mercury vapor electric discharge lamp by which the adhering strength of a fluorescent material film and a total beam of light can be improved as well as a beam of light maintaining ratio is increased and a less amount of blacking is caused and a method of manufacturing the low pressure mercury vapor electric discharge lamp.